Has it ever occurred to you that this person you call your "best friend" is just off her rocker? That's not a real term but it seems to me you are choosing to believe there is something wrong with you. I don't think there is. I think you are just unhappy about your dad and naive and gullible where your choice of friends has gotten you. Real friends don't do what she is doing. And you are letting her brow beat you into thinking you need to self-harm. You go on with that stupid kind of thinking and you'll be as nuts as she is, so stop it!
You need to immediately drop her out of your life. Listen to head phones, read a book or go do homework or read a book in the library at lunch. You can sit by yourself but still smile and say hello to people and ask if they like to join you and chat for a minute.
It doesnt matter what she says about you. You know its not true and you don't need her to have a happy life. The same as you don't need a phone like your brothers. Just understand he has really sucky parenting skills and when you move away from home you never have to deal with him again and you can buy yourself a phone with a cute case and you also do not need him or his approval to have a good life.
What you need is for you to have belief in yourself and self-pride and know you are a great human being going through a tough patch. You are worthy of respect and have the guts to demand it where you can. Right now it might only be with the witch you call a friend and it might mean when you turn 18 from your dad. But believe in you and stop letting the world walk on you. Best of luck kiddo be strong!